Bright Areas on Ceres Suggest Geologic Activity

If you could
fly aboard NASA's Dawn spacecraft, the surface of dwarf planet Ceres would
generally look quite dark, but with notable exceptions. These exceptions are
the hundreds of bright areas that stand out in images Dawn has returned. Now,
scientists have a better sense of how these reflective areas formed and changed
over time -- processes indicative of an active, evolving world.

"The
mysterious bright spots on Ceres, which have captivated both the Dawn science
team and the public, reveal evidence of Ceres' past subsurface ocean, and
indicate that, far from being a dead world, Ceres is surprisingly active.
Geological processes created these bright areas and may still be changing the
face of Ceres today," said Carol Raymond, deputy principal investigator of
the Dawn mission, based at NASA's Jet Propulsion Laboratory in Pasadena,
California. Raymond and colleagues presented the latest results about the
bright areas at the American Geophysical Union meeting in New Orleans on
Tuesday, Dec. 12.

Different Kinds of Bright Areas

Since Dawn
arrived in orbit at Ceres in March 2015, scientists have located more than 300
bright areas on Ceres. A new study in the journal Icarus, led by Nathan Stein, a
doctoral researcher at Caltech in Pasadena, California, divides Ceres' features
into four categories.

The first group
of bright spots contains the most reflective material on Ceres, which is found
on crater floors. The most iconic examples are in Occator Crater, which hosts
two prominent bright areas. Cerealia Facula, in the center of the crater,
consists of bright material covering a 6-mile-wide (10-kilometer-wide) pit, within
which sits a small dome. East of the center is a collection of slightly less reflective
and more diffuse features called Vinalia Faculae. All the bright material in Occator
Crater is made of salt-rich material, which was likely once mixed in water. Although
Cerealia Facula is the brightest area on all of Ceres, it would resemble dirty
snow to the human eye.

More commonly, in
the second category, bright material is found on the rims of craters, streaking
down toward the floors. Impacting bodies likely exposed bright material that
was already in the subsurface or had formed in a previous impact event.

Separately, in
the third category, bright material can be found in the material ejected when
craters were formed.

The mountain
Ahuna Mons gets its own fourth category -- the one instance on Ceres where bright
material is unaffiliated with any impact crater. This likely cryovolcano, a volcano formed by the
gradual accumulation of thick, slowly flowing icy materials, has prominent bright streaks on its
flanks.

Over hundreds
of millions of years, bright material has mixed with the dark material that
forms the bulk of Ceres' surface, as well as debris ejected during impacts.
That means billions of years ago, when Ceres experienced more impacts, the dwarf
planet's surface likely would have been peppered with thousands of bright areas.

"Previous research has shown
that the bright material is made of salts, and we think subsurface fluid
activity transported it to the surface to form some of the bright spots,"
Stein said.

The Case of Occator

Why
do the different bright areas of Occator seem so distinct from one another?
Lynnae Quick, a planetary geologist at the Smithsonian Institution in
Washington, has been delving into this question.

The leading
explanation for what happened at Occator is that it could have had, at least in
the recent past, a reservoir of salty water beneath it. Vinalia Faculae, the
diffuse bright regions to the northeast of the crater's central
dome, could have formed from a fluid driven to the surface by a small
amount of gas, similar to champagne surging out of its bottle when the cork is
removed.

In the case of
the Vinalia Faculae, the dissolved gas could have been a volatile substance
such as water vapor, carbon dioxide, methane or ammonia. Volatile-rich
salty water could have been brought close to Ceres' surface through
fractures that connected to the briny reservoir beneath Occator. The lower pressure at
Ceres' surface would have caused the fluid to boil off as a
vapor. Where fractures reached the
surface, this vapor could escape energetically, carrying with it ice and salt
particles and depositing them on the surface.

Cerealia Facula must have formed in a somewhat different process,
given that it is more elevated and brighter than Vinalia Faculae. The material at
Cerealia may have been more like an icy lava, seeping up through the fractures and
swelling into a dome. Intermittent phases of boiling, similar to
what happened when Vinalia Faculae formed, may have occurred during
this process, littering the surface with ice and salt particles that formed the
Cerealia bright spot.

Quick's
analyses do not depend on the initial impact that formed Occator. However, the
current thinking among Dawn scientists is that when a large body slammed
into Ceres, excavating the 57-mile-wide (92-kilometer-wide) crater, the impact
may have also created fractures through which liquid later emerged.

"We also see fractures on other solar system
bodies, such as Jupiter's icy moon Europa," Quick said. "The
fractures on Europa are more widespread than the fractures we see at
Occator. However, processes related to liquid reservoirs
that might exist beneath Europa's cracks today could be used as a
comparison for what may have happened at Occator in the past."

As Dawn continues the final phase of its mission, in
which it will descend to lower altitudes than ever
before, scientists will continue learning about the origins of
the bright material on Ceres and what gave rise to the enigmatic features in
Occator.

The Dawn
mission is managed by JPL for NASA's Science Mission Directorate in Washington.
Dawn is a project of the directorate's Discovery Program, managed by NASA's
Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible for
overall Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed
and built the spacecraft. The German Aerospace Center, Max Planck Institute for
Solar System Research, Italian Space Agency and Italian National Astrophysical
Institute are international partners on the mission team. For a complete list
of mission participants, visit: